This invention relates to a method and apparatus for the devolatilization of synthetic resinous thermoplastic material.
It is well known that the removal of volatile constituents, such as water, solvents, unreacted monomers, low molecular weight polymers and inert gases, from polymer compositions is a necessary step in preparing and processing polymer compositions. A wide variety of devolatilization methods and apparatuses have been employed to remove volatile materials from thermoplastic resin compositions, including extrusion from an extruder having venting orifices, extrusion under vacuum, falling film devolatilizers and the like. One conventional devolatilizer consists of a boiling heat exchanger which is situated over a vacuum chamber and a gear pump or screw pump to expel the polymer from the bottom of the vacuum chamber.
In the past, it has been very difficult to devolatilize polymers in a continuous process without excessive time at high temperatures which can cause the polymer to degrade in color, molecular weight and overall physical properties. While the conventional type of devolatilizer described above does allow for clean and complete disengagement of vaporized monomer and solvent from the polymer, it does not always allow a useful undegraded polymer product to be recovered in a reasonable period of time.
A particularly troublesome polymer is a rubber-modified high impact interpolymer of styrene and maleic anhydride. When this polymer is devolatilized in a conventional devolatilizer, the polymer does not flow into the pump rapidly enough and the devolatilizer soon fills with polymer and must be shut down for cleaning. If the temperature is increased above about 260.degree. C., the polymer will flow more rapidly but becomes severely degraded both in color and physical properties. In addition to a decrease in molecular weight of the rigid phase, the required "rubber" nature of the particles is lost.
Devolatilizing extruders have also been extensively tested and they have fewer problems with polymer breakdown since the mechanical action of the screw controls inventory time. Screw extruder devolatilizers equipped with venting orifices provide a large heat transfer surface wit good energy distribution and constantly expose fresh polymer surfaces. Due to the great amount of shearing involved, there is shear breakdown of the rigid phase molecular weight and reduction of the rubber particle size. While this can in some select cases still produce a polymeric product with acceptable properties, it does not, and cannot be made to, do an adequate job of disengagement of polymer and vapor. In spite of all efforts, vapor lines become fouled with accumulated polymer. This involves frequent shutdown for cleaning. This necessitates the shutting down of the entire polymerization line which takes about a day to restart and bring back to operating conditions. In addition, plugging of the vent orifices also can cause thermal decomposition and contamination of polymer. The installment of "on-line" sewer cleaning equipment reduces the problem of vacuum line plugging, but does not eliminate it.